Originally posted by cdma77
I was wondering why everthing is a trapazoid for fin shapes. I am planning on having a 3 inch diamter frame, 67" total length with nose cone. My total weight would be about 4.6 lbs with parachute and electronics. The motor should add about another 1.5 lbs. I was planning on using 6"x6" right angle triangle fins. I tried to understand Rocsim but some of the paramters are confusing me. Does anybody have any idea of this will be stable? I had a formula that I found that said to calculate fin size to take .17*(d+.5)*L. Based on that my fin size would be 9"x9" which sounds crazy ! Please help me out !
Thanks,
Jeff
All else being equal, it's because of drag. Seen from the top, the area presented to the oncoming air is larger the farther the fins stick out from the body. With triangular fins, the part that sticks out away from the body the most has relatively little control area (are that can be presented for stabilization when the bird starts to tilt). That is, at some point along the span (how much it sticks out) you go from gaining foremost in flight control to gaining foremost in cross-sectional area (ie. drag). If you figure out where that point of diminishing returns is, and cut the fin off there, guess what shape you have.
There's another issue that crops up in airplanes, but I haven't seen much on in rocketry, and that's fin tip vortices. Fin/wing tips cause a small horizontal tornado-like disturbance. This adds a fair amount to drag. Recently designers have added small wingtip fins like little rudders to the wings to reduce this. Airlanes need long wings, and so this is the best fix. Rockets don't, so clipping them makes more sense. Here's a bit about wing drag and vortices:
https://www.aero.ses.soton.ac.uk/courses/ME162/WebDBT/Demonstration.htm
As for stability testing, remember the cut-out method, included in this fine rocket stability primer paper:
www.rockets4schools.org/education/ Basic_Rocket_Stability.pdf